1. Field of the Invention
The invention relates to atomic force microscopes and, more particularly, relates to a method and apparatus for measuring energy dissipation during operation of an atomic force microscope such as during interaction between the probe and a sample scanned by it.
2. Discussion of the Related Art
Atomic Force Microscopes (AFMs) operate by scanning a probe over a surface using a high resolution three axis scanner, usually creating relative motion between the probe and the sample while measuring the topography or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980. AFMs typically include a probe, usually comprising a very small cantilever which is fixed at one end and which has a sharp probe tip attached to the opposite end. The probe tip is brought very near to or into contact with a surface to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample surface is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al, or some other deflection detector such as strain gauges, capacitance sensors, etc. Using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
Typical of microscopes employing oscillation of a probe tip to measure properties of a sample are those operating in the intermittent contact mode, including those operating in the "TappingMode" and the "light TappingMode" ("Tapping" and "TappingMode" are trademarks of Veeco Instruments, Inc.) and those operating in the magnetic mode. AFMs operating in the Tapping and light Tapping modes detect changes in probe oscillation due to contact with the sample surface to obtain an indication of the topography or other properties of the sample surface. AFMs operating in the magnetic mode, usually known as "magnetic force microscopes" or "MFMs", employ a magnetic probe tip and detect changes in the probe oscillation that are due to magnetic interaction between the tip and sample.
There are two primary ways of studying a mechanical system upon interaction of that system with another system (such as upon interaction of an AFM with a sample): either (1) forces or (2) energy. Considering the name of the field with which this invention pertains (atomic force microscopy), it is not surprising that the dominant approach to treating the interaction between a probe tip and a sample is to consider the forces resulting from this interaction. For instance, recent modeling of TappingMode AFMs has focused on addressing a differential equation of motion for the cantilever which includes a non-linear term to account for the tip sample interaction and then solving that differential equation, usually numerically. Although this force-based approach has historically worked quite well, it has the disadvantage of requiring the explicit solution of equations of motion.
The significance of taking an energy-based approach to the study of AFMs can be appreciated if one thinks of the probe/sample interaction in terms of energy imparted to the sample or another medium of interest by the probe tip (or taken away from the probe tip by the medium of interest) per interaction. In an AFM operating at a 300 kHz cycle and dissipating one picowatt of power per cycle, only 1/300,000th of a joule of energy is dissipated per cycle or about 21 ev per cycle. The energy of the bonds between atoms or molecules of relatively hard crystalline materials is generally about 5 ev. Hence, sufficient energy is dissipated to a sample during typical operation of an AFM to break between four and five atomic bonds of a hard substance. This dissipated energy is distributed between the probe tip and the sample in a proportion depending upon, among other things, the contact area between the probe tip and sample and the relative strengths of the bonds connecting the molecules of the probe tip and the molecules of the sample. Measuring energy dissipation during operation of an AFM could prove useful in a variety of scenarios.
For instance, measuring energy dissipation can provide information about probe tip wear and even reduce it. Probe tip wear is undesirable because, as a probe tip wears, it becomes more blunt and increases the contact area between the tip and the sample. This increased contact area is counterproductive because AFMs typically seek to maximize resolution by minimizing contact area between the probe tip and the sample. The composition of the probe tip (and, accordingly, the strength of the bonds connecting the molecules of the probe tip to one another) generally is known. If it is known that the molecular bonds of a probe tip will begin to break if the energy dissipation due to probe tip/sample interaction is above, for instance, 5 ev per cycle, then the operation of the AFM can be controlled to maintain the energy dissipation per cycle below that level, thereby reducing undesired probe tip wear and enhancing the performance of the AFM.
Measuring energy rather than or in addition to measuring force can also be useful in the study of relatively elastic substances such as polymers. The molecules of many polymers exhibit a so-called "visco-elastic" quality in that they have elasticity (i.e., they return to their original shape after being stretched within limits) and they also have substantial internal friction that dissipates energy when they are elastically deformed. For instance, a rubber band will become noticeably warmer when it is stretched. In the case of AFM operation in intermittent contact mode, the polymer material deforms and dissipates energy every time the probe tip contacts it during intermittent contact. The dissipated energy comes from the kinetic energy lost when the probe tip interacts with the polymer surface. Measuring energy dissipated by the probe tip during this interaction therefore can provide an indication of visco elasticity of polymers.
Measuring energy dissipation is also useful when studying biological samples because it can provide an indication of the type(s) of bonds present in the samples. Molecular bonds in many biological systems can be thought of as "lock and key" interactions. One molecule (the key) fits precisely into another molecule (the lock). It is possible to prepare an AFM to have a "key" molecule on the probe tip and to prepare a sample to have a "lock" molecule on its surface. Then, "lock and key" bonds will form every time the probe tip approaches the sample. These bonds then break as the probe tip moves away from the sample, thereby dissipating energy.
Energy dissipation can also be useful when operating an MFM. That is, during operation of an MFM, the magnetic field emanating from the probe tip will magnetize the sample as the probe tip approaches the sample surface. Energy dissipation necessarily accompanies this induction. Measuring this energy dissipation can provide a separate indication of the sample property.
The above examples are not intended to constitute an exhaustive list of applications in which measuring energy dissipation during AFM probe operation is desirable but instead to illustrate that measuring such dissipation can be used to acquire a wealth of information about a probe, a sample, or both that might not be readily available simply by measuring forces during probe operation.